The Basics of Air Compressor On a Ship

A compressor is one such device which is used for several purposes on a ship. The main aim of the compressor, as the name suggests, is to compress air or any fluid in order to reduce its volume. A Compressor is a multipurpose device which finds many applications on a ship. Some of the main forms of compressors used on ships are the main air compressor, deck air compressor, AC compressor and refrigeration compressor. In this article, we will learn about air compressors and its types.

Applications Of Air Compressors

Air compressor is a device with vast application in almost all sorts of industries and household requirements. In the maritime industry as well, air compressors are used in essential equipment or the feeder equipment to different systems. They can be employed in a number of processes ranging from small process of cleaning of the filters to the bigger and crucial tasks such as starting main as well as secondary engines.

Air Compressor produces pressurized air by decreasing the volume of air and in turn increasing its pressure. Different types of air compressors are used according to the usage.

In a more technical language, an air compressor can be defined as a mechanical device in which electrical or mechanical energy is transformed into pressure energy in the form of pressurized air.

Air compressor works on the principles of thermodynamics. According to the ideal gas equation without any temperature difference, with an increase in gaseous pressure, its volume reduces. The air compressor works on the same principle on which it produces compressed air, by reducing the volume of air this reduction in volume results in an increase in air pressure without any temperature difference.

air compressor

Types of Air Compressors

General Classification:

Air compressor on ships can be classified into two different types namely:

Main air compressor: These air compressors are high-pressure compressors has a minimum pressure value of 30 bars and used to run the main engine.

Service air compressor: It compresses air to the low pressure of only 7 bars and later used in service and control airlines.

Classification of Compressors As Per Design And Working Principle:

There are mainly four types of compressors:

  1. Centrifugal compressor
  2. Rotary vane compressor
  3. Rotary screw compressor
  4. Reciprocating air compressor

However, on ship reciprocating air compressor is widely used. A reciprocating air compressor consists of a piston, connecting rod, crankshaft, wrist pin, suction valve and discharge valves.

The piston is connected to the low and high side of the suction line and discharge line. The crankshaft rotates which in turn rotates the piston. The downward moving piston reduces the pressure in the main cylinder, the pressure difference will open the suction valve.

The piston is taken down by the rotating crankshaft and the low-pressure air gets filled in the cylinder. Now the piston reciprocates upward and this upward movement starts building up the pressure and closes the suction valve.

When the air gets pressurized to its specific value discharge valve gets opened and the pressurized air starts moving through the discharge line and gets stored in air bottle.

This pressurized air in air bottle can be used to run the main as well as auxiliary engines later on. There can be single acting and a double-acting reciprocating air compressor on the ship.

Rotary Screw Air compressor

Classification On The Basis Of The Usage 

Normally, air compressors onboard ships are:

  • main air compressor
  • topping up compressor
  • deck air compressor
  • Emergency air compressor
  • Main air compressor

Main air compressor: It is used to supply highly pressurized air to start main and secondary engines. Air compressor has an air storage bottle which stores the pressurized air. There are available different capacity main air compressors, but this capacity should be adequate for starting the main engine. The minimum air pressure required is 30 bars to start the main engine. A pressure valve is provided which reduces pressure and supplies controlled air from the storage air bottle. The control air filter controls the input as well as output air in the air bottle.

Topping up compressor: This type of compressor is used to cater to any sort of leakage in the system. It implies that if there observed any leakage in the system the topping up air compressor covers up the leakage by taking the lead. With leakage in system, the air pressure declines below the requisite level which can be replenished to its specified level by topping up compressor by supplying pressurized air.

Deck air compressor: Deck air compressor is used for deck use and as a service air compressor and might have a separate service air bottle for the same. These are lower capacity pressure compressors as the pressure required for service air is in between the range of 6 to 8 bar.

Emergency air compressor: An emergency air compressor is used for starting the auxiliary engine at the time of an emergency or when the main air compressor has failed for filling up the main air receiver. This type of compressor can be motor driven or engine driven. If the motor is driven, it should be supplied from an emergency source of power.

Efficiency of Air Compressors

The air compressors can work efficiently if installed properly as per the installation guide. All the crew available should be responsive in working with an air compressor in an emergency as these are the basic part of almost all important machinery systems on the ship. Efficiency of the air compressor can be increased by following techniques and installations.

Pressure-Bar: Pressure-bar or pressure gauge should be installed within all the compressors to ensure that the air pressure and discharge the air at the specified pressure. Without this device if air is pressurized below the requisite value it cannot propel or run the system to which it is employed.

Safety Devices: These are the devices used to reduce the loss of energy from the air compressor and increase efficiency. Safety devices automatically shut down the input and output air when adequate compressing is reached and saves the device from overpressure.

Main Components Of Air Compressor

Some of the important components of air compressor which are common to all the available types of compressors are briefed as under:

  1. Electricity or power source: This is the key component of any type of compressor and essential for running the compressor. Power source or electric motor is used to run the compressor efficiently and at a constant speed without fluctuations.
  2. Cooling Water: Cooling water is used to cool the compressor in between different stages.
  3. Lubricating Oil: Lubricating oil is necessary to keep the all mobile parts of the compressor lubricated. This lubrication reduces the friction in parts of the compressor and thus imparts more life to the compressor by reducing wear-tear of components of the compressor.
  4. Air: This is the component without which an air compressor can’t be even imagined. The air around us is in low pressure and served as an input to the compressor.
  5. Suction Valve: Suction valve is provided with a suction filter it inputs the air, which is to be compressed in the main compartment of the compressor.
  6. Discharge Valve: This valve takes the output air to discharge at a requisite location or to the storage tank or storage air bottle.


Working Of Air Compressor  

Air compressor consists of air bottle which can store compressed air at a specified pressure. The compressor compresses air and stores this pressurized air in air bottle. When this pressurized air injected to the engine through air gun or any other equipment, it kicks the propeller and runs the engine. 

Uses of Air Compressor on Ship

Onboard a ship, compressed air is used for several purposes. On the basis of application, different air compressors are kept for a particular usage.

  • Air compressor is used to provide the starting air to various machines and main engine.
  • Other than the main engine other systems also require compressed air. These systems are Control valves. Throttle controls and other monitoring systems which work on pressurized air.
  • This compressed-air controls many operations in the auxiliary engine as well.
  • In pneumatic tools like cleaning, devices compressed air is required to keep the devices running and serve the purpose efficiently.
  • In whistling operation of ships also compressed air is employed and the fog horns operate on the compressed air.
  • Hydraulic jack in the ship also uses compressed air to perform lifting operations.
  • Many times boilers; refrigerants and heat exchangers in ship are started using compressed air.
  • Sometimes compressed air is used in kicking the propellers of the ship manoeuvring system.

In a nutshell, the compressor is the mechanical device which runs on the principles of thermodynamics that reduce the volume of air and increases the pressure of air.

This highly pressurized air when injected to run either main engine or auxiliary devices such as heat exchanger; boilers; etc.

The most common type of compressor used in the marine industry are double acting reciprocating air compressors. There are multiple compressors provided on a ship to serve different purposes. They can run main and auxiliary engines.

Sometimes propeller of the ship runs on the compressed air which increases the application of air compressor in the marine industry.

Air compressors can never be forgone from the ship as they have wide applications on board from smaller tasks of cleaning the filters to the crucial process of running the engine and even propelling the ship.

You may also like to read – Compressed Air Line On Ships – A General Overview

Disclaimer: The authors’ views expressed in this article do not necessarily reflect the views of Marine Insight. Data and charts, if used, in the article have been sourced from available information and have not been authenticated by any statutory authority. The author and Marine Insight do not claim it to be accurate nor accept any responsibility for the same. The views constitute only the opinions and do not constitute any guidelines or recommendation on any course of action to be followed by the reader.

The article or images cannot be reproduced, copied, shared or used in any form without the permission of the author and Marine Insight. 

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  1. I am an engine room cadet on my first voyage. I will like to know more about the air compressor… Thank you sir

  2. Hi! I am a marine pilot. I would like to enquire on air pressure requirement in term of kick start during manoeuvring. I google and observed the following; “Total air receiver capacity is to be sufficient for Twelve (12) starts of Reversible engines and six (6) starts for non- reversible engines”. The above is a international requirement? Where can I refer the above said requirement of main air receiver.

    Thank you.

  3. You may not get a detailed information on this. ABS rules , probably others too, mention that there should be compressed air available for 12 starts of reversible and 6 starts for unidirectional engines. This is taking into account of starting failures for various reasons including the expertise and skill of the operator. How much is the pressure required for start would have been documented in the manuals.
    It is an international requirement. Google and experienced chief engineers, masters etc might be helpful.

  4. I want to know why we have compress air system on ballast tank on tankers. the diagram said it is for bubbling. i want to know why we need bubbling on ballast tank. is there any involve some rules and regulation or if this is mandatory

    thank you for reply my e-mail is

  5. @Arman Ponce
    Anti-Ice-Systems in ballast water tanks
    According to regulations from Germanic Lloyd, all ships of ice-class E3 must be equipped with a device which prevents freezing of ballast water tanks that reach above the water line.
    Each relevant tank is equipped with an air bubble system, which circulates the water within them to prevent freezing.

  6. Hi, I would like to ask what is the common pressure for service air and control air onboard ships

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